You should contact two people:
1) the director of graduate studies, either for the department you want to study in, or the university. Write to the administrator for your prospective department, and ask them if there is a faculty member who manages graduate studies. If there is, write to them; if not, then write to the general one for the university.
2) If your GPA is high enough, then write to a professor you are interested in studying with. Don't ask them outright to be your supervisor -- say that you are interested in their field, and would like to know if they are currently accepting graduate students.